What do you mean exactly? Are you asking how to set the order in which an image is drawn? If so, images are drawn from back to front in the order that you draw them. So if you draw bitmaps 1, 2, 3, and 4, the result will be that 1 is in the far "background" and 4 is in the "front". Is that what you meant?

The above draws a coin (represented by a yellow circle) and slowly moves it up and down. The vertical movement is dictated by the coin's Y value (coin_y), which is updated every frame in the update phase.